In general, this invention relates to laser-aided direct material deposition processes and, more particularly, to a system and method of monitoring residual stresses during laser-aided direct material deposition.
During the process of laser-aided direct material deposition for cladding or fabrication of a product, residual stresses develop in the product. Residual stress accumulation leads to distortion and premature failure of the product during its use or operation. When the accumulated residual stresses exceed the yield strength of the material, cracking occurs during the fabrication process. Thermal expansion and sometimes phase transformation are the main contributors to residual stress. Most steels, for example, change from austenite with face centered cubic structure (FCC) to martensite with body centered tetragonal crystal structure (BCT) above a certain critical cooling rate. The specific volume of BCT is 4% higher than that of FCC, and therefore martensitic transformation produces considerable stress. Clearly, management of stress accumulation during the direct material deposition process is critical for the production of non-defective components with close tolerances and precise dimensions.
There are several techniques for post mortem, i.e., after fabrication or after failure, measurement of stress accumulation, but these techniques are not timely and do not save the product. To take corrective action, it is necessary to monitor the stress accumulation during the process. It is easier, however, to monitor strain and calculate stress using Hooke""s law or other applicable stress-strain relationship for the material of the product. A method for measuring strain generated by direct material deposition has recently been reported by Mazumder et al., xe2x80x9cRapid Manufacturing by Laser Aided Direct Deposition of Metalsxe2x80x9d, Proceedings of Powder Metallurgy Conference, July 1996, Vol. 15, pp. 107-118. This method utilizes a substrate especially designed for making test specimens. It has a middle thin section on top of which the deposition occurs, and two thick end sections, which remain bolted down during the laser deposition process. Two strain gauges are mounted on the back side of the specimen, i.e. the side opposite the material deposition side, in two orthogonal directions, X and Y. After one layer is deposited, the bolts are loosened, releasing the specimen, and then the strain is measured, from which the residual stress accumulation for the first layer can be calculated. From this residual stress value, the approximate number of layers required to exceed yield strength can be calculated. This method, useful as it may be, does not provide timely information to avoid frequent process interruptions for the purpose of relieving the build-up of residual stresses.
Background for the laser-aided direct metal deposition process can be found in xe2x80x9cLaser Material Processingxe2x80x9d, W. M. Steen, 1998 Springer, and in U.S. patent application Ser. No. 09/107,912, filed Apr. 10, 1997, which is incorporated herein by reference.
The invention is directed to a system and method for monitoring and controlling in real-time the development of residual stress accumulation in a product during its fabrication by a laser-aided, computer-controlled, direct material deposition process. The system includes a laser source, a laser controller and a numerical controller. The material is deposited on a substrate which is equipped with strain gages on its back surface, i.e. the surface opposite to the surface on which material is deposited. The strain gages measure real-time changes in chosen locations and directions, typically in two orthogonal directions along and across the deposition direction. The strain gage measurements are sent to a computer with a stress analysis software package, which uses the strain data as an input to calculate the stress accumulation. The computer is programmed to determine whether critical conditions, such as a pre-determined fraction of the yield strength, are reached at any location. Based on this information, the computer sounds a warning signal, or directs a command to the laser controller to adjust the parameters of the laser beam or the laser process, or directs the laser numerical controller to discontinue the process.
In addition to the conventional strain gages, which will be referred to as purely mechanical strain sensors, sensors operating on other principles are employed to obtain measurements for determining in real time stress accumulation during the deposition process. Examples are acoustic sensors, which detect sound waves emitted by the product undergoing strain changes or phase transformations, and optical sensors operating on interferometric principles to detect strain. All non purely-mechanical sensors are first calibrated against purely-mechanical sensors. Multiple inputs from different type of sensors are integrated in a computer program, which performs a meta-analysis or fusion of data and determines the confidence level of the residual stress prediction based on the sensor inputs.